The processing of citrus fruits such as grapefruit and Natsudaidai fruit to yield fruit juices is complicated by several factors which affect the bitternesses of final products. It would be advantageous for processors to simultaneously remove two or more bitternesses from fruit juices.
In recent years, the art of immobilized enzymes has been developed to facilitate extremely efficient and economic enzyme use and permit the design of continuous enzymatic processes for fruit juice debittering.
In some juices, naringin, a main bitter component of several citrus juices, is present in excess of 700-800 ppm amounts definitely shown to be responsible for making the juice too bitter. Naringin can be hydrolyzed by naringinase which is an enzyme mixture containing .alpha.-rhamnosidase and .beta.-glucosidase, in which naringin is hydorlyzed by the .alpha.-rhamnosidase to rhamnose and prunin and then by the .beta.-glucosidase to glucose and naringenin. Since prunin bitterness is less than one third that of naringin, only the first hydrolyzing activity, i.e. the .alpha.-rhamnosidase activity, is essential for debittering of citrus juices such as grapefruit juices and Natsudaidai juices.
In order to control the naringin contents in citrus juices, Aspergillus niger naringinase which contains both .alpha.-rhamnosidase and .beta.-glucosidase activities has been immobilized on various insoluble carriers, such as copolymers of maleic anhydride with styrene, hollow fiber, porous glass beads, DEAD-Sephadex A25, chitin and tanninaminohexyl cellulose; see, e.g., Int. J. Biochem., 2, 448-456 (1971), J. Food Sci., 44, 1358-1361 (1979), J. Ferment. Technol., 57, 310-316 (1979), J. Ferment. Technol., 55, 493-500 (1977), Nat. Sci. Council. Monthly R.O.C., 9, 871-881 (1981), J. Ferment. Technol., 62, 263-267 (1981) and Agric. Biol. Chem., 42, 1849-1853 (1978).
However, these immobilized enzymes are not suitable for use in the continuous processes for debittering fruit juices due to poor operational stabilities.
It is disclosed by Jimeno A. et al. in Process Biochem., 22, 13-16 (1987) that Penicillium sp. naringinase immobilized on glycophase-coated porous glass is used for natural grapefrurt juice debittering. However, the column reactors made of these immobilized enzymes have column blocking or filtering action when natural fruit juice is continuously passed through it. Therefore, juice clarification by pectinase treatment or centrifugation is required before the juice debittering.
It is known from Process Biochem., 7, 9-12 (1972) and Methods in Enzymology, 44, 227-243 (1976) that column blocking or filtering action of catalyst bed can be avoided when the enzyme is entrapped in cellulose triacetate.
Moreover, cellulose mono-acetate gel bead column is disclosed in J. Sci. Food Agric. 1981, 32, 1183-1190 for the removal of the bitter limonin only from citrus juices.
It is accordingly an object of the present invention to demonstrate that Penicillium sp. naringinase immobilized on cellulose triacetate can simultaneously remove naringin and limonin from fruit juices, especially citrus juices.
A further object of the present invention is to provide a process for simultaneous removal of naringin and limonin from fruit juices.
Another object of the present invention is to provide higher operational stability for successively debittering fruit juices for a long time.
These and other objects will readily become apparent to those skilled in the art in the light of the teaching herein set forth.